A typical evaporative emission control system in a modern automotive vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the fuel tank. During conditions conducive to purging, the canister is purged to the engine intake manifold by means of a canister purge system that comprises a canister purge solenoid valve that is operated by an engine management computer. The canister purge valve is opened in an amount determined by the computer to allow the intake manifold vacuum to draw vapors from the canister through the valve into the engine.
U.S. governmental regulations require that certain future automobiles that are powered by volatile fuel such as gasoline have their evaporative emission control systems equipped with on-board diagnostic capability for determining if a leak is present in a portion of the system which includes the fuel tank and the canister. One proposed response to that requirement is to connect a normally open solenoid valve in the canister vent, and to energize the solenoid when a diagnostic test is to be conducted. A certain vacuum is drawn in a portion of the system which includes the tank headspace and the canister, and with the canister and the tank headspace not being vented due to the closing of the canister vent, a certain loss of vacuum over a certain time will be deemed due to a leak. Loss of vacuum is detected by a transducer mounted on the fuel tank. Because of the nature of the construction of typical fuel tanks, a limit is imposed on the magnitude of vacuum that can be drawn. Too large a vacuum will result in deformation and render the measurement meaningless. In order to avoid this problem, a relatively costly vacuum transducer is required. Since typical automotive vehicles are powered by internal combustion engines which draw intake manifold vacuum, such vacuum may be used for performance of the diagnostic test, but typically this requires that the engine be running in order to perform the test.
The invention disclosed in commonly assigned allowed application Ser. No.: 07/770,009, filed Oct. 2, 1991, now U.S. Pat. No. 5,191,870 issued Mar. 9, 1993, provides a solution to the leak detection problem which is significantly less costly. The key to that solution is a new and unique vacuum regulator/sensor which is disposed in the conduit between the canister purge solenoid and the canister. The vacuum regulator/sensor is like a vacuum regulator but with the inclusion of a switch that is used to provide a signal indicating the presence or the absence of a leak. A diagnostic test is performed by closing the tank vent and using the engine manifold vacuum to draw, via the canister purge solenoid valve and the vacuum regulator/sensor, a specified vacuum in the tank headspace and canister. Upon the requisite vacuum having been drawn, the vacuum regulator/sensor closes to trap the drawn vacuum. If unacceptable leakage is present, a certain amount of vacuum will be lost within a certain amount of time, and that occurrence causes the switch of the vacuum regulator/sensor to give a signal indicating that condition.
U.S. Pat. No. 5,146,902 discloses a diagnostic system and method for evaluating the integrity of a portion of the canister purge system that includes the tank and canister by means of positive pressurization rather than negative pressurization (i.e., rather than by drawing vacuum). In certain canister purge systems, such a diagnostic system and method may afford certain advantages over the system and method described in the aforementioned commonly assigned allowed patent application.
For example, certain types of leaks, for example cracked hoses and faulty gas caps, may be more susceptible to successful detection. Moreover, the evaporative emission control system may be diagnosed either with or without the automobile's engine running. One means to perform positive pressurization of the fuel tank's headspace and the canister is a devoted electric-operated air pump, which can be of quite simple construction, and therefore relatively inexpensive. If the vehicle already contains a source of suitably pressurized air, that could constitute another means, thereby eliminating the need for a separate devoted pump. Another means for performing positive pressurization of the tank's headspace is a vacuum-actuated, electrically controlled pump. If such a pump is actuated by engine intake manifold vacuum, then the engine must be run to perform the test.
A further benefit of positive pressurization over negative pressurization is that the increased pressure suppresses the rate of fuel vapor generation in the tank, and such attenuation of fuel vapor generation during a diagnostic test reduces the likelihood that the test will give, under hot weather conditions which promote fuel vapor generation, a false signal that would erroneously confirm the integrity of the canister and tank whereas the same test during cold weather would indicate a leak.
According to the disclosure of U.S. Pat. No. 5,146,902, atmospheric air is pumped directly into the fuel tank's headspace where it is entrained with fuel vapor that is already present. Concern has been expressed about pumping air directly into the fuel tank particularly if for some reason the pump continued to pump beyond the time when it should have shut off. Overpressurization of the tank headspace and vapor collection canister may create atypical pressures and/or air-fuel ratios in the canister/tank headspace. One possible consequence of overpressurization is that some fuel vapor may be forced out the atmospheric vent of the canister.
The present invention relates to a means for introducing the pumped air into the evaporative emission system that can alleviate the tendency toward such consequences; specifically it relates to introducing the pumped air into the evaporative emission system through an atmospheric vent port of the canister after that port has been closed to atmosphere by the closing of a canister vent solenoid (CVS) valve through which the canister is otherwise vented to atmosphere during non-test times.
Should the air pump continue to run for any reason after a diagnostic test has concluded, the pumped air will not be forced into the tank headspace. The pumped air will not even enter the canister, but rather will be returned to atmosphere through the CVS valve which re-opens at test conclusion to relieve the tank test pressure.
The canister contains an internal medium that collects fuel vapors so that the vapors do not pass to the atmospheric vent port. During a diagnostic test, air pumped into the canister vent port must pass through that medium before it can enter the tank headspace, and consequently it is fuel vapor laden air, rather than merely air alone, that pressurizes the tank headspace.
Further specific details of the construction and arrangement of the inventive system, and of the method of operation thereof, along with additional features and benefits, will be presented in the ensuing description.
Drawings accompany this disclosure and portray a presently preferred embodiment of the invention according to the best mode presently contemplated for carrying out the invention.